1. Field of the Invention
The present invention relates to a gas-discharge display apparatus that can display color images.
A plasma display panel (PDP) that is a typical display device having a screen emitting light by gas discharge is becoming widely available as a wide screen display for a television set after the color display thereof has been succeeded in commercialization. One of the challenges to improve the image quality of PDPs is to enhance the reproducible color range.
2. Description of the prior art
As a color display device, an AC type PDP having three-electrode surface discharging structure is commercialized. This type has a pair of main electrodes for sustaining, which are arranged in parallel for each line (row) of the matrix display, and an address electrode for each column. Division walls for preventing interruption of discharge between cells are provided in stripes. A surface discharging structure includes a substrate on which the pairs of main electrodes are arranged and an opposing substrate on which a fluorescent layer for color display is arranged, so that deterioration of a fluorescent layer due to an ion impact upon discharge can be reduced to obtain a longer life. The “reflection type” that has the fluorescent layer on the back substrate is superior to the “transparent type” that has the fluorescent layer on the front substrate concerning light emission efficiency.
In general, Penning gas containing neon (Ne) and a trace of xenon (Xe) (4-5%) is used as a discharging gas. When the discharge between main electrodes occurs, the discharging gas radiates ultraviolet rays, which excite the fluorescent material to emit light. Each pixel includes three cells for red (R), green (G) and blue (B) light colors, and the display color is decided by controlling the light intensity of the fluorescent material of each color. Conventionally, the composition of the fluorescent materials and the ratio of light intensities of three colors are selected so that the display color becomes white when each of the red, green and blue color light intensities is set to the maximum within the variable range.
Concerning the composition of the discharge gas, many studies have been done, a three-component gas (Ne, Xe and He) that is a mixture of the above-mentioned Penning gas and helium, a two-component gas containing helium and xenon (He and Xe), and another three-component gas containing helium, argon and xenon (He, Ar and Xe) are known well.
As explained above, since the fluorescent material emits light by gas discharge in a PDP, the mixing of the light color of the discharge gas into the light color of the fluorescent material cannot be avoided. This causes a problem of deterioration of the color reproducibility.
FIG. 13 shows a light emission spectrum of a two-component gas containing neon and xenon. FIG. 14 is a chromaticity diagram showing influences of the neon light emission on color reproduction.
As shown in FIG. 13, a plurality of light emission spectrums appears in the visible light wavelength range above 580 nanometers. The peak of the light emission of the discharge gas (585 nanometers) is adjacent to the maximum light emission peak (590 nanometers) of the red fluorescent material. Therefore, orange color due to the light emission of the discharge gas is added regardless of the color reproduced by the fluorescent material, so the reddish display occurs over the entire screen. In FIG. 14, the inside of the triangle of the solid line connecting the color coordinates of the respective fluorescent materials, plotted with small rectangles, is the reproducible color range when the color of the gas light emission is not added. In FIG. 14, the inside of the triangle of the broken line is the color reproducible range of the PDP measured in a darkroom. The real color reproducible range is narrowed compared with the original color reproducible range. Especially, reproducibility of blue and green colors is inferior. Concerning the red color, the reproducibility is not so deteriorated since the wavelength of the gas light emission is approximate to that of the light emission of the fluorescent material. However, focusing on the light color of the fluorescent material, the red fluorescent material is different from the ideal red (620 nanometers) defined in the NTSC system. Namely, even if the influence of the gas light emission is little, it is still necessary to improve the color purity of red color. At present, there is no red fluorescent material that emits light of ideal red color and satisfies other use conditions such as efficiency of exciting ultraviolet rays and life. Green and blue fluorescent materials can emit light of substantially ideal color.
Since the display capacity of blue color is degraded by the gas light emission, the display color of white pixels has a low color temperature compared with the color reproduced by the fluorescent materials of three colors. It is difficult to optimize the relative light intensities of red, green and blue colors in consideration of relative luminosity factor by combining materials because there are few kinds of fluorescent materials that satisfy the use condition at present. Therefore, the color temperature of the white color is low compared with a CRT even if the color of the gas light emission is not added. Furthermore, if the color of the gas light emission is added, the color temperature drops further. More specifically, the color temperature of the white color display is 5,000-6,000 K under the condition of the same amplitude signal applied for red, green and blue colors, while the color temperature of a CRT for a TV set in Japan is above 10,000 K. Therefore, it is necessary to raise the color temperature of the white color. However, since the color temperature has different optimal values depending on the use of the display, the region (the country) where the display is used or other factor, it is preferable that the color temperature can be selected easily within the range of about 6,000-12,000 K.
The color temperature can be raised by weakening the relative light intensities of green and red colors to blue color. The conventional method for adjusting the color temperature and disadvantages thereof are explained below.
(1) Adjustment by Fluorescent Material:
There is a method for adjusting the relative light intensities of red, green and blue colors by selecting the fluorescent materials, a forming shape thereof (i.e., the shape in which the fluorescent material is found) or the forming area thereof (i.e., the area on which the fluorescent material is formed). This method substantially reduces the brightness of the panel since the light intensities of green and red fluorescent materials are weakened compared with that of the blue fluorescent material that is usually lacking in intensity. Furthermore, there is a limited selection of materials as mentioned above. The adjustment by the forming shape has low reproducibility. If the cell size of blue color is increased to enlarge the forming area thereof, the margin of the voltage to be applied is narrowed and the display becomes unstable since the display characteristics depend on the cell size. In addition, manufacture of panels having different light intensities of the fluorescent material in accordance with the use and the region (country) of use may deteriorate the productivity.
(2) Adjustment by Signal:
The intensity balance between the video signals of red, green and blue colors is adjusted. In an example where the level of the video signal is represented by eight bits, i.e. 0-255, the maximum intensities of blue, green and red are represented by 255, 200 and 180, respectively so as to display white color of the maximum intensity. Thus, the color temperature of white color is raised by reducing the amplitude of the input signal of green and red colors compared with that of blue color. This method reduces the brightness of the panel in the same manner as the method (1) mentioned above, and degrades the capacity of gradation display of green and red colors compared with that of blue color that can be displayed in 256 steps of gradation.
Another problem about the color temperature is that contrast in a well-lighted room is low. The contrast in a well-lighted room (hereinafter, referred to as the bright-room contrast) means a ratio of intensity of light emitted by the fluorescent material and intensity of external light reflected by the PDP. In general, PDPs have a large reflection ratio of external light and a small value of the bright-room contrast. It is clear that the bright-room contrast will be improved by raising the light intensity of the panel and reducing the reflection ratio of external light, but it is not easy to satisfy the compatibility between them. For example, improvement of the filter for EMI measure is considered. Usually, the front surface of the PDP is provided with a filter having transmittance of 40-70% over the entire region of visible light wavelength for protecting interference of electromagnetic field. Though the light emitted inside the panel passes through the filter only once, the external light passes through the filter twice, once each in both directions. Therefore, the filter improves the bright-room contrast. If the filter having less transmittance is used, the bright-room contrast is further improved. However, since the improvement of the color temperature by the above-mentioned method will reduce the light intensity of the panel, the filter having low transmittance cannot be used for improving the bright-room contrast.